Method of operating mercury cathode electrolytic cell plant

ABSTRACT

Describes a process of operating a flowing mercury electrolysis cell plant in which a portion of the depleted brine is recycled to the cells without going through a resaturation and purification step and in which brine is introduced at several points along the cells and baffles are provided at intermediate points to direct flow of the brine into the spaces between the anodes and the flowing mercury cathode.

Unite States Patet [56] References Cited UNITED STATES PATENTS [72] Inventors OronzioDeNora;

Giovanni Trisoglio, both of Milan, Italy mflwm .t eae t o a l b y ie a TJSK 2008 4666 9999 HHHH 7888 770 3373 1 ob on 4599 2 ,3 2333 i .m .m m n h E i t n .m n. m I m 0 0 ww e d N m. 7. ns 8r m m a 2M OS 0. de mm e.l p mmfi AFPA NMM 2247 [iii FOREIGN PATENTS Milan, Italy Continuation of application Ser. No. 626,860, Mar. 29, 1967, now abandoned. This application Mar. 27, 1970, Ser. No. 20,872

Assistant Examiner-R. L. Andrews Attorney-Hammond & Littell [54] METHOD OF OPERATING MERCURY CATl-IODE ELECTROLYTIC CELL PLANT 6 Claims, 10 Drawing Figs.

ABSTRACT: Describes a process of operating a flowin g merlectrolysis cell plant in which a portion of the depleted is recycled to the cells without going through a resaturation and purification step and in which brine is introduced at several points along the cells and baffles are provided at intermediate points to direct flow of the brine into the s between the anodes and the flowing mercury cathode.

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ORONZIO Do NORA GIOVANNI TRISOGLIO I ATTORNEYS METHOD OF OPERATING MERCURY CATHODE ELECTROLYTIC CELL PLANT 6This application is a continuation-in-part application of copending application, Ser. No. 626,860, filed March 29, 1967 now abandoned.

This invention relates to electrolysis cell plants and a method of operating the same. The invention will be described specifically with reference to the operation of mercury cell plants for the electrolysis of aqueous salt solutions such as, for example, sodium and potassium chloride. It will be understood, however, that the invention is not limited to the specific embodiments herein described or the specific salt solutions herein used for purposes of illustration.

Electrolysis cells of the type which may be used in the practice of this invention are described in U. S. Pats. Nos. 2,544,138, granted March 6, 1951 and 2,958,635, granted Nov. 1, 1960. These cells, generally designated as horizontal mercury cells, consist of an enclosed elongated trough sloping slightly toward one end. The cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows along the bottom of the cell toward the lower end. The anodes are generally composed of flat rectangular plates of graphite suspended from graphite or copper rods in such a manner that their flat surface is spaced a short distance above the flowing mercury cathode and substantially parallel therewith. The body and sides of the trough are generally steel with a corrosion resistant lining of hard rubber, concrete, stone or other nonconducting material.

In the operation of this type of cell, the electrolyte, which may be brine or an aqueous solution of any electrolyte compound which upon electrolytic decomposition will give the products desired, is introduced at the upper end of the cell and flows toward the lower end. For example, a solution of sodium chloride is electrolyzed in such a cell. Electric current passes through the solution between the anodes and the mercury cathode. When sodium chloride is the electrolyte, chlorine is fonned at the anodes and passes to the top of the cell and out through openings in the cell cover which are provided for this purpose. Sodium is formed at the cathode and amalgamates with mercury. The sodium amalgam is withdrawn at the lower end of the cell, cycled to a decomposer packed with graphite where it is contacted with water to form sodium hydroxide, hydrogen and mercury. The mercury is recycled to the cell for reuse as the cathode.

A system of partition walls extending vertically at a short distance from both cell ends divide the trough into three compartments: the intermediate compartment, in which are assembled the anodes and electrolysis is performed; the upper end compartment, which receives the inflowing mercury stream; and the lower end compartment, from which the outflowing amalgam stream is discharged to the decomposer.

The partition walls, or weirs, delimiting the upper end compartments, are so arranged as to allow mercury to spread evenly over the cell width prior to entering the electrolysis compartment, while building up a liquid seal that prevents brine and chlorine from leaking out through the gaps left between the partition walls and the cell bottom. A similar function is performed by the partition walls delimiting the lower end compartment. This division of the cell is illustrated in US. Pat. No. 2,544,138.

ln cells of this type the distance between the graphite anodes and the mercury cathode is very important. This distance should be as small as possible to reduce consumption of energy, but if this distance is too small, a short circuit with the cathode or unwanted side reactions will take place. The posts of electrically conducting material from which the anodes are suspended extend through the cell cover and are fastened to suitable devices whereby it is possible to adjust the spacing between anode and cathode. However, the graphite anodes are consumed in operation, so that, in order to maintain the proper spacing, it is necessary to provide for periodic adjustment.

While brine is flowing from one end of the cell to the other, its temperature increases, as a consequence of the resistance opposed by the electrolyte to the passage of current, as well as on account of other irreversible phenomena bringing about anodic overvoltage and causing greater wear on the anodes in one end of the cell with reference to the others.

If kept within certain limits, such temperature rise is an advantage, since, other conditions being equal, it involves a decrease in electrolyte resistivity and anodic overvoltage, and hence a saving in energy consumption. On the other hand, the higher the temperature, the higher the rate of attack of the electrolyte and of the reaction products, notably chlorine, on the cell lining and the anode material.

The increasing wear on the anodes as temperature increases, is enhanced by the gradual decrease in salt concentration that accompanies the temperature rise as the electrolytic reaction proceeds.

For these reasons the maximum temperature that brine is allowed to reach usually does not exceed 75 C. l 67 F.

It has heretofore not been possible to maintain the electrolyte concentration and temperature in mercury cathode electrolytic cells within narrow limits because of the structure of these cells.

The elongated and narrow shape of the mercury cell trough hinders the inner remixing of the electrolyte stream, which would homogenize the temperature and concentration throughout its path. As a consequence thereof, along the cell length a temperature and a concentration gradient exists, which are both undesirable, since the maximum concentration attainable in the feed brine corresponds to the saturation point and the feed temperature must be kept well below the maximum acceptable value, in order not to exceed the latter at the other points within the cell and in particular at the outlet end. Moreover, the higher the temperature and concentration gradients, the wider will be the difference in wear rate suffered by the anodes from one end of a cell to the other, so that it becomes more difficult to keep satisfactory operating conditions, while shutdown periods for cell maintenance and anode replacement become even more frequent and time consuming.

The difficulties outlined above also present a major obstacle to increasing the current density in mercury cathode electrolysis cells because by raising the current density, other conditions being equal, the concentration and temperature gradients will also be raised. On the other hand, the trend in this industry is to ever higher cell loads, so as to improve unit capacity and thereby reduce investment costs.

In order to reduce some of the difficulties listed above, it has been proposed to increase the brine flow rate in the closed loop circuit made up by the electrolysis cells and the brine resaturating and treating plant, so as to keep the temperature rise and concentration drop in the electrolyte within narrower limits. Such procedure, however, requires a larger size resaturating and treating plant and a bigger consumption of treatment chemicals and larger dimensions for ancillary equipment, such as settlers and filters, whose value represents a very appreciable fraction in the overall plant cost.

One of the objects of this invention is to provide a new method and means to reduce the temperature rise and the concentration drop of the electrolyte throughout a mercury cathode electrolysis cell while decreasing the throughput of the electrolyte in the brine treating plant.

Another object of this invention is to provide adequate means to distribute the brine inside the cell and promote its renewal in the narrow zone between the anode and cathode surfaces.

Another object of this invention is to increase the operating efficiency of mercury cathode electrolysis cell plants by operating the cells within narrower limits of temperature and electrolyte concentration than heretofore possible.

Another object of the invention is to reduce the wear differential on anodes in different parts of the cell.

Another object of the invention is to reduce the cost of electrolyte preparation by recycling a large portion of the electrolyte back through the cells without passage through the bodiment of the invention,

FIG. 4 is a perspective elevation, with parts broken away, of a typical type of mercury cathode electrolytic cell embodying means to be applied in connection with this invention,

FIG. 5 is a cross-sectional view of the cell taken along the line 5-5 of FIG. 4,

FIG. 6 is a partial cross-sectional side view along the line 6-6 of FIG. 5,

FIG. 7 is a graph representing the temperatures and concentrations of the electrolyte at different equidistant sampling points along the electrolysis compartment during a run performed under usual conditions of operation as heretofore practiced, that is, without direct recycling of any depleted brine fraction FIG. 8 is a similar graph taken during a run in which depleted brine was recycled according to the embodiment of this invention illustrated in FIG. I and under operating conditions as specified in the following example 1,

FIG. 9 is a similar graph taken during a run performed with depleted brine recirculation according to the embodiment of this invention illustrated in FIG. 2 and under operating conditions as specified in the following example 2, and

FIG. 10 is a similar graph taken during a run perfonned with depleted brine recirculation according to the embodiment of this invention illustrated in FIG. 3 and under operating conditions as specified in the following example 3.

In the embodiment of this invention illustrated in FIG. 1 the electrolysis section, which is usually formed by a large number of flowing mercury cathode electrolysis cells, is represented for sake of simplicity, by only two cells A and B. While the mercury and amalgam circuits are usually as many as there are cells, and independent of one another, the feed brine is introduced into the several cells from a common header C, connected with each cell by means of a branch pipe D. According to normal practice, as followed heretofore, the feed brine header C conveys brine coming from the resaturation and purification section E after being pumped into a head tank F. In the resaturation section the depleted brine is fortified by passing through a bed of solid salt and then is submitted to a chemical treatment, followed by settling and filtration, to remove the soluble impurities present in the solid salt. In order to prevent free chlorine dissolved in depleted brine from harmfully interfering with these operations, the resaturation must be preceded by thorough dechlorination which is achieved by acidifying the depleted brine stream and degasifying it in a flash tank, after which the residual chlorine is removed by inert gas bubbling and possible reaction with a chemical reducing agent in a dechlorinator G. These operations are complex and impose additional economic burdens on the construction and operation of electrolysis plants of this type so that the recycled flow rate through the resaturation and purification plant has to be kept down to the minimum value compatible with a satisfactory operation of the electrolysis process. Such value, in the majority of industrial applications, and in the case of sodium chloride, corresponds to a residual salt content in depleted brine of not less than 270 grams per liter, and to a concentration downstream the saturators close to 3 l0 gJl.

In accordance with this invention it is possible to increase the recirculating brine flow rate throughout the electrolysis cells and thus diminish the temperature and concentration gradients therein, even without overloading the brine saturating and purifying treating plant. To achieve this purpose, a fraction of the depleted brine leaving the cells A and B is directly recycled to the feed header F by means of a bypass line H, and is mixed in the header tank F with the fortified brine stream 1 coming from the resaturating and treating plant E.

As illustrated in FIG. I, the mixing of the two streams H and I may conveniently take place in a common header tank F, although such a procedure is not to be considered a limitation to the object of the present invention. A heat exchanger I may be conveniently arranged on the bypass line H for the feed brine, whereby the latter may be cooled down by means of an appropriate coolant, so as to control the temperature in the mixed feed brine stream. Under particular circumstances, such as during startup, the heat exchanger .I may be used for heating, rather than cooling, the recycled stream H, until the desired operating conditions are attained. Other heating or cooling means may be provided on the brine stream H to be submitted to fortification and purification. Pumps for circulating the brine streams and for recirculating the mercury from the denuders to the upper end of cells A and B have been illustrated diagrammatically In the alternative embodiment illustrated in FIG. 2 the mixed brine feed stream C is subdivided into a number of streams D and D, which in this example are two, but could be more. One of these, D, is fed, as usual, at the upper end of cells A and B, while the other stream D is introduced at an intermediate point, suitably selected, in the electrolysis compartment.

In the alternative embodiment illustrated in FIG. 3, the feed brine is subdivided into two streams D and D' which are introduced at the end and at an intermediate point of the cells A and B. However, whereas in the embodiments of FIGS. 1 and 2 both streams were composed. of mixed brine of the same composition, in the example of FIG. 3, the stream entering the inlet end of cells A and B is a mixture of fortified and depleted brine from the lines H and I, while the inflow D at the intermediate point of the cells consists of saturated brine I, which is supplied from a separate head tank F. By this arrangement it is not only possible to further improve the temperature and concentration distribution of the electrolyte throughout the cell length, but also to make available a dechlorinated and saturated brine feed from the line I at the inlet end, whenever required. This is needed, in particular, on shutdown in order to quickly displace from the cell, as soon as it is deenergized, the chlorinated brine stream, which otherwise would react with mercury and dissolve it as a chloride complex. The switching of the saturated brine stream 1' from the intermediate point to the inlet end, while cutting off the mixed brine feed, may be expediently achieved by opening the bypass valve K while shutting off valves L and M.

A typical mercury cell which might be used in the practice of this invention is illustrated in FIGS. 4, 5 and 6. It will be understood, however, that other type cells may be used. The cell illustrated consists of a trough 1 of steel with a slightly sloped bottom on which the mercury cathode layer (not shown) flows. Each side and end of the trough terminates at the top with a flange 3 and the walls of the trough are protected from chemical attack by an insulating lining 2. The anodes comprise horizontal graphite plates 6 attached to vertical cylindri cal graphite rods 7, which in turn are provided with metallic conductor bolts 8 for attachment to the positive bus bars 12 and for attachment to the anode supporting frame structure.

The anodes 6 are suspended and supported from a metal frame spider which consists of transverse bars 5 and 11 secured to a longitudinal beam 24. The transverse bars 5 are adjustably supported on posts 4, so that the height of the spider, and of the anodes supported therefrom, can be adjusted relative to the cell trough. Eyes 240 are provided on the beam 24 so that the entire anode structure can be lifted from the cell trough when necessary.

Anode support bars 11 are attached to the beam 24 at spaced intervals and the anodes are suspended from the bars 1 I by connector bolts 8 and nuts 10.

A flexible cover 9 made of sheet rubber or other sheet plastic material, through which the anode connections 8 protmde, closes the top of the cell and is fastened to the sides of the cell by means of pressing strips 14 and clamps 13.

The sheet forming the flexible cover 9 is pressed down against the anode rods by discs 23 to prevent escape of gases and chlorine outlets 15 are provided through the flexible cover.

The details of the above-described arrangement are embodied in U. 5. Pat. No. 2,958,635 and have been recited here only in order to better distinguish the improvements hereinafter described.

A set of feed brine conduits, which in FIG. 4 are shown in the number of two and designated as 16, 160, are distributed at several points of the cell length. Each conduit comprises a measuring device 17, preferably embodying means for automatic flow rate control, and a valve-l8 for manual regulation of the brine flow.

Each feed conduit 16 extends inside the cell, above the anode plate into a sparger 19, consisting of a foraminous pipe suspended horizontally and crosswise to the trough.

Since the depleted brine recycling method of this invention permits considerably larger feed rates than otherwise possible without such recycling, the linear velocity of the brine fluid stream is considerably increased; it is thus possible to attain a state of hydrodynamic conditions such as to enhance the electrolyte renewal and the displacement of gas bubbles from the narrow spacing between the anode and the cathode surface, in that the forced convection inherent in the higher impressed velocity can now cooperate with the natural convection generated by depletion of the brine and by the rising tendency of the gas bubbles generated at the anode surfaces. In order to facilitate the penetration of the liquid stream into the interelectrodic gap, vertical baffle plates 20 are arranged crosswise and downstream each incoming brine line and preferably also at other points suitably spaced along the cell trough. The baffle plates 20 are fitted within the gap separating one transverse bank of anode plates from the next, so that the lower edge of the baffle 20 will reach a level close to the lower anode faces. The baffle plates are preferably situated in contact, or nearly so, with the downstream anode plate bank, so as to leave the free space between the banks completely available for the downward motion of brine and convey it into the interelectrodic gap as illustrated by arrows 20a in FIGS. 4 and 6.

The displacement of gas bubbles by forced convection is facilitated if the slots 21, which cut into the anode lower surface, are arranged lengthwise, rather than crosswise of the anodes and holes 22 are drilled between each slot 21 and the upper face of the anode plates 6 to ease out the gas bubbles escaping upward by natural convection.

The baffle plates 20 are preferably rigid and made of, or lined with, an insulating material. The plates 20 may be secured in place in any suitable manner, in the particular embodiment exemplified in FlGS. 4, S and 6, the baffle plates 20 are bent at the upper end so as to form horizontal flanges 20b provided with extensions resting on the top flanges 3 of the trough and fastened between the trough lining 2 and the flexible cover 9, in fluidtight relationship therewith. The baffle plates 20 may be secured directly on the anodes preferably on the upstream side of each bank of anodes.

Example 1 In a mercury cell plant, loaded at 100,000 A, temperature and concentration measurements were carried out at several points of the brine circuit, while the totality of the brine flow was passed through the saturators and the chemical treating plant and back to the cells. The values thus detennined were the following:

Feed rate to each cell 6.250 l./hr. NaCl concentration at cell inlet 308 g./l. NaCl concentration at cell outlet 272 g./l. Concentration drop 36 g./l. Cell inlet temperature 55C.

Temperature and concentrations gradients were both practically constant (to within :1 C.) throughout the cell length, as shown by the graph, FIG. 7, which shows temperature and concentration measurements taken at five sampling points equally spaced along the cell trough.

A bypass line for the depleted brine was then provided, in accordance with FIG. 1, so as to recycle a fraction H of the depleted brine directly to the feed head tank F, by means of a lifting pump and through heat exchanger J. The overall brine flow rate was increased over the former value by 50 percent and one-third of the depleted brine stream was directly recycled while the remainder was passed through the saturation and purification plant so that the load through the saturating and treating plant was the same as formerly. The new steady state conditions were as follows:

9,370 Llhr.

Feed rate to each cell NaCl concentration at cell inlet 296 g./l. NaCl concentration at cell outlet 272 g./I. Concentration drop 24 g./l. Cell inlet temperature 59 C. Cell outlet temperature 72 C. Temperature rise 13 C.

Temperature and concentration measurements taken at the five sampling points within the cell are plotted in F IG. 8. By comparison with FlG. 7, the lesser deviation from the mean value thus obtained becomes evident. The amount of depleted brine recycled directly to the cells is not limited to one-third but can be larger.

Example 2 The cells were fed, as in the preceding example, with a mixture consisting by two-thirds of concentrated and purified brine and one-third of depleted brine directly recycled. Instead of feeding the whole stream to the cell inlet end, it was split into two equal parts, which were introduced into the inlet end and the midpoint of the cell trough respectively, according to the embodiment of FIG. 2. The new steady conditions were the following:

Feed rate to inlet end 4685 l/hr. Feed rate to midpoint 4685 l/hr, Overall feed rate to each cell 9.370 |./hr. NaCl concentration at cell inlet 296 g./l. NaCl concentration at cell outlet 272 g./l. Concentration drop 24 g./l. Cell inlet temperature 58.5 C. Cell outlet temperature 72.0 C. Temperature rise l3.5 C.

Temperature and concentration values throughout the cell length are plotted in FIG. 9. This and the above-noted results clearly evidence the further reduction obtainable in the maximum deviations from the mean values.

Example 3 Feed rate through cell inlet end 5,320 Llhr. Feed rate through cell midpoint 4,050 l./hr. Overall feed rate to each cell 9,370 l./hr. NaCl concentration in the saturated stream 308 g./l. NaCl concentration in the mixed stream 287 g./l. NaCl concentration at outlet end 272 g./l. Concentration drop between cell inlet and outlet l5 gJl.

inlet end temperature C Outlet temperature 72 C. Temperature rise 1 l C.

These results and the graph of FIG. show the further improvement thus obtainable in temperature and concentration distribution throughout the cell.

it should be understood that the foregoing examples are not to be considered as establishing any limitations to the scope of the invention. The fraction of depleted brine being directly recycled to the feed line, as well as the splitting of this stream and of the saturated stream, or the number of the feed points into the cell, may be varied at will, without thereby departing from the object of the invention. By the same token, the cells composing an electrolysis circuit, or the several feed points in one cell, may be provided singly or by groups with independent bypass lines and heat exchangers without thereby departing from the invention and the principles of the invention may be applied to other type cell plants than the ones specifically illustrated herein. The term salt solutions" as used in the following claims is intended to include all salt solutions capable of undergoing decomposition by electrolysis in cells of the type described.

What is claimed is:

l. The method of operating a horizontal flowing mercury cathode electrolysis cell plant having mercury cells with anodes and a substantially horizontal flowing mercury cathode and means to electrolyze salt solutions therein which comprises flowing a concentrated salt solution into and through said cells, electrolyzing said salt solution in said cells removing the gaseous electrolysis products from the cells separate from the depleted salt solution, removing a depleted salt solution separate from the gaseous electrolysis products from said cells, recycling at least one-third of said depleted salt solution back to the cells, and recycling the remainder of the salt solution through a dechlorinating chamber and through a resaturating and purification plant and back to the cells, combining said recycled depleted salt solution stream and said saturated salt solution stream before recycling back to the cells, introducing one portion of said combined salt solutions into an end of said cells and another portion at an intermediate point between the ends of said cells.

2. The method of claim 1, in which the amount of recycled depleted salt solution is between one-third and nine-tenths of the total flow of salt solution through said cells.

3. The method of claim 1, in which the recycled salt solution is caused to flow downward under the anodes and between the anodes and cathode at intermediate points between the ends of said cells.

4. The method of claim 1, in which the total flow of salt solution through said cells is increased by about 50 percent over the flow normally used without recycle of said depleted salt solution back to said cells.

5. The method of claim 4, in which a portion of the saturated salt solution is mixed with the recycled depleted salt solution and introduced into said cells and a portion of the saturated salt solution not mixed with recycled depleted salt solution is introduced into said cells.

6. The method of claim 5, in which the mixed stream of recycle depleted salt solution and saturated salt solution is introduced into an end of said cells and the saturated salt solution not mixed with recycle depleted salt solution is introduced into an intennediate point between the ends of said cell.

i i 18 t i 

2. The method of claim 1, in which the amount of recycled depleted salt solution is between one-third and nine-tenths of the total flow of salt solution through said cells.
 3. The method of claim 1, in which the recycled salt solution is caused to flow downward under the anodes and between the anodes and cathode at intermediate points between the ends of said cells.
 4. The method of claim 1, in which the total flow of salt solution through said cells is increased by about 50 percent over the flow normally used without recycle of said depleted salt solution back to said cells.
 5. The method of claim 4, in which a portion of the saturated salt solution is mixed with the recycled depleted salt solution and introduced into said cells and a portion of the saturated salt solution not mixed with recycled depleted salt solution is introduced into said cells.
 6. The method of claim 5, in which the mixed stream of recycle depleted salt solution and saturated salt solution is introduced into an end of said cells and the saturated salt solution not mixed with recycle depleted salt solution is introduced into an intermediate point between the ends of said cell. 